Processes for cracking hydrocarbon feeds with hot regenerated fluidized catalytic particles are known generically as "fluid catalytic cracking" (FCC).
Distilled feeds such as gas oils are preferred feeds for FCC. Such feeds contain few metal contaminants and make less coke during cracking than heavier feeds. However, the higher cost of distilled feeds provides great incentive to use heavier feeds, e.g., residual oils, as feed in FCC. Resids generally contain more metals, which poison the catalyst and an abundance of coke precursors, asphaltenes and polynuclear aromatics, which end up as coke on catalyst rather than cracked product. Resids are also hard to vaporize in FCC units. FCC operators are well aware of the great difficulty of cracking resids and of the profit potential, because these heavy feeds are much cheaper than distilled feeds.
Most FCC operators that crack resid simply blend in a small amount of resid, on the order of 5 or 10 wt %, with the distilled feed. This blending of different feeds, and addition of the blended feed to the base of the riser, is the way most refiners operate, but some units operate with split feeds, i.e., cracking different kinds of feed at different elevations in an FCC riser.
U.S. Pat. No. 4,422,925-Williams et al. taught an FCC process with a light feed fed to the base of a riser, and a worse feed, having a higher tendency to form coke, charged higher up the riser.
U.S. Pat. No. 4,218,306-Gross et al., having a common assignee with the present invention and incorporated by reference, taught cracking gas oils in a lower part of a riser then cracking a more difficult feed, such as a coker gas oil, in an upper section of the riser.
Blending, or split feeds with a heavier feed added higher up in the riser, are not completely satisfactory when the feed contains large amounts of resid or asphaltenics which are difficult to vaporize quickly in the base of a riser reactor.
Most units cracking resids stay with the blended feed approach and try to improve the process by using relatively large amounts of atomizing steam. Thus while conventional FCC units, cracking wholly distillable feeds, might add 1 or 2 wt % steam with the heavy feed to improve atomization, those units cracking heavier, more viscous feeds add significantly more steam, 3, 4, or 5 wt % steam, or even more. While increased atomization steam usually improves cracking efficiency, it also substantially increases the load on the main column, and limits primary feed throughput. Steam reduces hydrocarbon partial pressure, which is beneficial, but increases overall pressure, which increases operating costs. The increased steam usage associated with cracking resids also produces large amounts of sour water which is a disposal problem.
Some units have tried to deal with residual feeds by charging the resid containing feed to the base of the riser, cracking it momentarily at an unusually high temperature, then quenching with a heat sink such as water or a lower boiling cycle oil higher up in the riser. The higher temperatures were believed sufficient to thermally shock asphaltenes into smaller molecules which could then be cracked catalytically. Thus U.S. Pat. No. 4,818,372, taught the advantages of quenching quickly, with an auxiliary fluid within one second and preferably in less than half a second. The '372 process would improve the cracking of residual feeds but caused some problems.
The '372 process used large amounts of quench, either large amounts of water or even larger amounts of a recycled fluid such as a cycle oil. Water quench increases plant pressure and sour water production, much as does increased use of atomizing steam. LCO or HCO quench does not create as severe a pressure problem as water, because of smaller molar volume, but there is some loss of riser cracking capacity and a significantly increased load on the main column.
We realized that although large amounts of quench increased conversion of heavy stock, the large amounts of quench had offsetting side effects.
We conducted experiments and ran computer model studies to try to determine more accurately what was happening in the riser, and to learn more precisely why riser quenching led to improved cracking of resid containing stocks, with a view to developing a better process.
We discovered that cracking of heavy chargestocks in a riser reactor could be completely characterized by considering only thermal and catalytic reactions.
Based on our understanding of what went on in the riser, we made computer simulations of various quench scenarios, and discovered that quick quenching was not preferred, and that quenching after 1 second, and preferably after about 2 seconds of vapor residence time, gave improved results.
Our simulations showed that while quenching within one second is good, delayed quench was even better. Thus quenching within 0.5 seconds of vapor residence time increased gasoline yields, but quenching at 1.5 seconds made gasoline yields better still. Delaying quenching until after 2.0 seconds of vapor residence time further improved gasoline yields.
The capital and operating expenses associated with quenching somewhat higher up in the riser, after 1.5 or 2.0 seconds, are essentially nil. The improved gasoline yields are, however, significant.
We found that the conventional teachings on riser quench could be ignored. We could use an unconventional amount of quench (less than normal) in an unconventional place (higher up in the riser) and get results essentially equivalent to those using larger amounts of quench within 1.0 seconds of vapor residence time. We could get the benefits of quenching, while minimizing the unpleasant side effects of the conventional approach to quenching.
Using another approach, we found that using conventional amounts of quench in an unconventional place (higher in the riser) gave increased conversion of fresh feed and increased gasoline selectivity.
We also found a way to let the energy contained in the quench stream perform useful work in reducing riser pressure. We could add large volumes of steam to a riser in a way which would increase riser pressure only slightly or even decrease riser pressure. This reduced pressure, or lesser increase in pressure from steam addition translates into increased plant capacity and/or increased gasoline selectivity.